The ANSS event ID is ak020da7vc4e and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020da7vc4e/executive.
2020/10/15 16:05:15 61.173 -149.324 34.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/10/15 16:05:15:0 61.17 -149.32 34.7 4.1 Alaska
Stations used:
AK.BRLK AK.CUT AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN AK.HOM
AK.KNK AK.M20K AK.P23K AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AK.SWD AT.PMR AV.SPU AV.STLK TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.84e+22 dyne-cm
Mw = 4.11
Z = 46 km
Plane Strike Dip Rake
NP1 160 70 -65
NP2 286 32 -139
Principal Axes:
Axis Value Plunge Azimuth
T 1.84e+22 21 231
N 0.00e+00 23 331
P -1.84e+22 58 104
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.95e+21
Mxy 9.05e+21
Mxz -1.87e+21
Myy 4.77e+21
Myz -1.29e+22
Mzz -1.07e+22
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--########-------------------#######
-###########---------------------#####
-############----------------------#####
#############------------------------###
###############------------------------###
###############-------------------------##
################------------ ---------##
#################----------- P ----------#
################----------- ----------
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##### #########---------------------
#### T ##########-------------------
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Global CMT Convention Moment Tensor:
R T P
-1.07e+22 -1.87e+21 1.29e+22
-1.87e+21 5.95e+21 -9.05e+21
1.29e+22 -9.05e+21 4.77e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201015160515/index.html
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STK = 160
DIP = 70
RAKE = -65
MW = 4.11
HS = 46.0
The NDK file is 20201015160515.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 80 85 10 3.15 0.1645
WVFGRD96 2.0 40 40 -90 3.40 0.2006
WVFGRD96 3.0 275 50 40 3.40 0.2085
WVFGRD96 4.0 270 50 30 3.41 0.2042
WVFGRD96 5.0 70 50 -30 3.44 0.2195
WVFGRD96 6.0 270 80 45 3.44 0.2358
WVFGRD96 7.0 275 40 35 3.47 0.2528
WVFGRD96 8.0 280 35 40 3.55 0.2694
WVFGRD96 9.0 280 40 40 3.57 0.2856
WVFGRD96 10.0 280 55 45 3.59 0.2961
WVFGRD96 11.0 280 55 45 3.61 0.3047
WVFGRD96 12.0 280 55 45 3.62 0.3095
WVFGRD96 13.0 280 55 45 3.64 0.3105
WVFGRD96 14.0 280 60 45 3.65 0.3105
WVFGRD96 15.0 280 60 45 3.66 0.3092
WVFGRD96 16.0 285 60 50 3.67 0.3062
WVFGRD96 17.0 285 60 50 3.68 0.3024
WVFGRD96 18.0 5 70 45 3.71 0.3002
WVFGRD96 19.0 0 75 50 3.72 0.3015
WVFGRD96 20.0 170 85 -45 3.72 0.3063
WVFGRD96 21.0 165 80 -50 3.74 0.3167
WVFGRD96 22.0 165 75 -55 3.75 0.3305
WVFGRD96 23.0 165 75 -55 3.77 0.3439
WVFGRD96 24.0 165 75 -55 3.79 0.3567
WVFGRD96 25.0 165 75 -55 3.80 0.3679
WVFGRD96 26.0 165 75 -55 3.81 0.3787
WVFGRD96 27.0 165 70 -50 3.83 0.3959
WVFGRD96 28.0 165 70 -50 3.84 0.4141
WVFGRD96 29.0 165 70 -50 3.86 0.4315
WVFGRD96 30.0 165 70 -50 3.87 0.4462
WVFGRD96 31.0 165 70 -50 3.88 0.4623
WVFGRD96 32.0 160 65 -50 3.89 0.4751
WVFGRD96 33.0 160 70 -55 3.91 0.4892
WVFGRD96 34.0 160 70 -55 3.92 0.5035
WVFGRD96 35.0 160 70 -55 3.93 0.5157
WVFGRD96 36.0 160 70 -55 3.94 0.5240
WVFGRD96 37.0 160 70 -55 3.95 0.5317
WVFGRD96 38.0 160 70 -55 3.95 0.5362
WVFGRD96 39.0 160 70 -55 3.96 0.5418
WVFGRD96 40.0 160 75 -65 4.07 0.5378
WVFGRD96 41.0 160 70 -65 4.07 0.5442
WVFGRD96 42.0 160 70 -65 4.08 0.5509
WVFGRD96 43.0 160 70 -65 4.09 0.5554
WVFGRD96 44.0 160 70 -65 4.10 0.5602
WVFGRD96 45.0 160 70 -65 4.11 0.5618
WVFGRD96 46.0 160 70 -65 4.11 0.5629
WVFGRD96 47.0 160 70 -65 4.12 0.5622
WVFGRD96 48.0 160 70 -65 4.12 0.5590
WVFGRD96 49.0 160 70 -65 4.13 0.5563
WVFGRD96 50.0 160 70 -65 4.13 0.5509
WVFGRD96 51.0 160 70 -65 4.13 0.5470
WVFGRD96 52.0 160 70 -60 4.13 0.5405
WVFGRD96 53.0 160 70 -60 4.13 0.5349
WVFGRD96 54.0 160 70 -60 4.14 0.5281
WVFGRD96 55.0 160 75 -60 4.14 0.5228
WVFGRD96 56.0 165 75 -60 4.14 0.5149
WVFGRD96 57.0 165 75 -60 4.14 0.5097
WVFGRD96 58.0 165 75 -60 4.14 0.5024
WVFGRD96 59.0 165 75 -60 4.14 0.4957
The best solution is
WVFGRD96 46.0 160 70 -65 4.11 0.5629
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3
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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00